Credit: Sue Stubbs
Credit: Paul Philipson
Credit: Sue Stubbs
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An aerial view of most major Australian cities shows only the occasional green patch, stark against an endless procession of grey concrete roofs, paths, car parks and bitumen roads.
As our climate grows inexorably hotter, it’s not unusual for city dwellers to experience daily temperatures more than 7°C higher than those living outside the built-up areas. And such temperature rises in our urban spaces can’t all be blamed on climate change. We have created urban heat islands in our cities by removing the best natural air-conditioning: plants. When we erect concrete jungles, we trap heat rather than regulating it through our flora.
Now gardeners are joining forces with architects to restore a little greenery to our cities by taking advantage of a little-used space – the roof.
“Interest in green roofs is growing substantially around the world,” says Australian landscape architect Jock Gammon.
Living, breathing air-conditioners
One of the most significant benefits of green roofs is their immediate cooling effect, which goes beyond the insulation properties of a layer of soil and plants.
“As the plant transpires water, it reduces surrounding air temperatures and cold air drops down the face of the building. With enough roof gardens, there can be a significant effect on cities,” says architect Paul Downton, who is the co-creator of Adelaide’s groundbreaking Christie Walk eco-development.
Gammon says that a green roof also enhances the performance of solar panels by reducing their operating temperature, making them more efficient.
Canada’s Ryerson University predicted that the city of Toronto could reduce its ambient air temperatures by up to 2°C if around eight per cent of the city’s buildings had green roofs – a total area of around 5,000 ha. They estimated a resulting annual energy use reduction of 114 MW and a corresponding saving in
greenhouse gas emissions of 56,000 tonnes.
Green roofs can also improve air quality. A 1996 NASA study found that a single fig tree purifies 10 cubic metres of air a day; other research showed that one square metre of grass removes about 0.2 kg of particles from the air each year.
In cities, clean rainwater quickly becomes dirty stormwater run-off, but this can be captured instead by green roofs. While plants release some water back into the atmosphere, the rest can be naturally filtered for use by building residents.
Downton names some further benefits: “As well as being quite beautiful, visually, green roofs bring back habitat and increase biodiversity in an area.”